Numerical Simulation Of Vibration Fatigue Of Welded Joints

Welded structures were widely applied in modern industrial fields, such as offshoreplatform, shipbuilding, vehicle engineering. Yet its quality affects the security applicationof various structures directly. Under the alternating load, vibration fatigue damage ofwelded joint can often be caused by resonance and led to loss of life and property easily.So vibration fatigue analysis of welded joints has important actual engineering significance.While a lot of manpower, materials and times are consumed and even some experiments oflarge welded structures are not achieved, wherefore simulation technology is veryimportant for stress and fatigue analysis of welded joints. Now simulation technology ismainly applied in static fatigue analysis of welded joints, but less is applied in vibrationfatigue of welded joints because of the lack of simulation parameters. So in present work,vibration stress distribution and vibration fatigue life prediction of butt joint and cross joint,which the materials were structure steel Q235 B and aluminum ally 5A06, were mainlyresearched by simulating technology.Welding residual stress and stress concentration are the key effective factors forfatigue property of welded joints. Under the cycle load, welding residual stress wasdifficult to determine by simulating methods because of relaxation behavior. Hence themethod of IIW was used to deal with the welding residual stress. But stress concentrationcould be considered by simulating methods. Based on the principles of finite elementmodel, solid models of expressing real structures with welds were selected and finiteelement models of butt joints and cross joint with 8mm(Q235B) and 10mm(5A06) werebuilt. According to the principle of partition mesh, structured and transitional mesh waselected and the meshes which were near the welds were optimized, so the total number ofelements and nodes were reduced. Used the FEA software ABAQUS, stress distribution ofwelded joints were simulated under unit load and stress concentration factors weredetermined in order to providing the relevant data for subsequent analysis. According tothe dynamic theory, the first 30 modals of welded joints in free were simulated and theresonance frequency ranges were determined by mode shapes and participation factors. Atthe same time, they provided a clear dividing line between static fatigue and vibrationfatigue.Feasibility of simulation technology was verified through simulation of static fatigue,because of the same damage mechanism between static fatigue and vibration fatigue. First,the fatigue tests of different welded joints were completed on the high frequency fatiguetesting machine. And the test results shown that the fracture location of joints basicallylocated in weld toe. Then, the test results were analyzed and three kinds of S-N curve weregotten. They were the test S-N curve that was with fitting test data, the recommendationS-N curve that was with fitting test data and m=3 of IIW recommendation and standardS-N curve that it came from IIW which was in accordance with the joint shape andmaterial. Later, static stress distribution of welded joints was simulated by the hot spotstress method. And the hot spot location and hot spot stress concentration coefficients weredetermined. Based on the stress analysis, the effect of the applied stress, the mean stress,the residual stress and stress concentration coefficients were considered by simulatingmethod. Under the test hot spot S-N curve, recommendation hot spot S-N curve andstandard hot spot S-N curve, the static fatigue lives were predicted. Comparing with thetest life values, the deviations were all less than 30 percents, so they could be used inpractice engineering. Among them, the predication deviation of recommendation hot spotS-N curve was greater and the predication deviation of standard hot spot S-N curve wasmore conservative. At the same time, it was proved that the numerical simulationtechnology had high reliability and it can be applied in vibration fatigue analysis.The relationship of the static fatigue S-N curve and the vibration fatigue S-N curvecould be determined by the applied frequency, because under the different frequency, theratio of the conditional fatigue limit and the fatigue strength coefficient was approximatelyequal, the ratio of the fatigue strength coefficient and the actual fracture strength wasapproximately equal,too. According to the logarithmic form of the fatigue S-N curve, therelationship of vibration fatigue S-N curve’s correction factor C2 and frequency effectivecoefficient was proposed. Based on Hooker’s law, tensile tests with different tensile strainrate were finished, the actual fracture strength of welded joints were determined. At last,the frequency effective coefficient and correction factor C2 were gotten. Through the staticfatigue S-N curves, the vibration fatigue S-N curves were deduced and they provided thenecessary parameter of vibration fatigue life prediction.The vibration fatigue analysis process of welded joints was determined based on thedynamic theory. First, by using the Frequency-Response method of ABAQUS andconsidering the effect of stiffness, mass and damping parameters, the stress distribution ofthe whole welded joints were simulated. The vibration stress distribution of welded jointswas received, and the dangerous parts were at the fixed side of weld toe, and themagnification of vibration stress was gotten. Second, combined with the vibration stressdistribution of welded joints, vibration fatigue S-N curve and Miner’s fatigue cumulativedamage rule, vibration fatigue lives of welded joints were predicted by the whole lifemethod of fatigue analysis software MSC.Fatigue. Comparing with test data, it proved thatthe life predicative values were located in the dispersion zone of test values. At last, FEmodel, software, FEM in this paper had high accuracy, so it could be used to analyzevibration fatigue of welded joints and direct practice engineering.